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You are over stocked by a large margin. How long has it been running/cycled & what is the filtration?

Stop messing with your pH please, You are doing your fish no favors. A stable ph is absolutely key but we will get back to that.

Do you have a liquid test kit that tests for ammonia & nitrItes?

What is your water change [WC] routine like?

EDIT: What are you using [I mean your test kit] to monitor your cycle on your 20?

Gas mileage isn't everythingOIIIIIIIOLack of planning on your part does not constitute an emergency on my part.Why pretend there are no stupid questions? Actually, There are many stupid questions: "Should I drink this bleach?" Is just one example.
Having said that, Just because it's a stupid question doesn't mean that it shouldn't be asked. It's better to know.A warm beer is better than a cold beer. Because nothing is better than a cold beer, and a warm beer is better than nothing.

I water chg about every 2 weeks, I am a novice but have had this tank stable several months...

Have strip tests for ammonia and quick strip but liquid ph, I'm terrified, asked a friend she said google so I found y'all! Haven't done anything in about 2 hours... Watching fish swim nuts a bit and then they look normal.

How long has this tank been running with that stock? I find it hard to believe it has been several months with no deaths from aggression.
Changing water every two weeks with that stock is probably the issue. Change more water now and don't add anything but dechlorinater to the water. The nitrate levels are probably through the roof, and if the tank isn't cycled so are the ammonia and nitrite levels.

If it's called tourist season why can't I shoot them?
Everything happens for a reason. Sometimes that reason is you are stupid and make bad decisions. I think my fish is adjusting well to the four gallon, He's laying on his side attempting to go to sleep on the bottom of the gravel.
A moderator on a fish forum should be able to identify an oscar... Don't you think?

Strips are notoriously inaccurate, The liquid tests would serve you far better.

Is there a reason you haven't tested the tank with issues for ammonia & whatever else they test for? Please do - That info is relative to your situ?

Gas mileage isn't everythingOIIIIIIIOLack of planning on your part does not constitute an emergency on my part.Why pretend there are no stupid questions? Actually, There are many stupid questions: "Should I drink this bleach?" Is just one example.
Having said that, Just because it's a stupid question doesn't mean that it shouldn't be asked. It's better to know.A warm beer is better than a cold beer. Because nothing is better than a cold beer, and a warm beer is better than nothing.

How many tanks are we discussing here? Stability won't cycle a tank. The 30 gallon is severely overstocked and needs a water change. Have you tested the ammonia, nitrite, and nitrate on it yet? Please test and let us know what the results are so we can help further.

If it's called tourist season why can't I shoot them?
Everything happens for a reason. Sometimes that reason is you are stupid and make bad decisions. I think my fish is adjusting well to the four gallon, He's laying on his side attempting to go to sleep on the bottom of the gravel.
A moderator on a fish forum should be able to identify an oscar... Don't you think?

Is there a reason you haven't tested the tank with issues for ammonia & whatever else they test for? Please do - That info is relative to your situ?

Heads up, You do not have anywhere near a big enough tank for koi. By that I mean it needs a pond to live in.

Gas mileage isn't everythingOIIIIIIIOLack of planning on your part does not constitute an emergency on my part.Why pretend there are no stupid questions? Actually, There are many stupid questions: "Should I drink this bleach?" Is just one example.
Having said that, Just because it's a stupid question doesn't mean that it shouldn't be asked. It's better to know.A warm beer is better than a cold beer. Because nothing is better than a cold beer, and a warm beer is better than nothing.